Adhesives can be made in many forms. Many adhesives are made from relatively high molecular weight polymers mixed with low molecular weight tackifying resins. They may or may not be further combined with other components such as oil or wax to control properties such as viscosity at application temperature, adhesive open time, and set time.
Polymers supply many of the basic adhesive properties such as cohesion and elongational and elastic behaviors. Relatively low molecular weight resins (usually with number average molecular weights (Mn) ranging from 300 to 2000) are useful in adhesive applications. When combined with polymers such as those described above, these low molecular weight resins decrease the entanglement density of the polymer chains, which improves the adhesive properties. Tackifiers also have relatively high glass transition temperatures (Tg) for such low molecular weight amorphous materials. The Tg is typically from below 0° C. to about 90° C. Tackifiers interact with the polymer chains and associate with the amorphous polymer phases at the desired operating temperatures; therefore, good compatibility and dispersion is desired for advantageous performance.
Other components are often used and dispersed in the adhesive matrix. Waxes are often used in hot-melt adhesives (HMAs) to lower viscosity at the application temperature and to decrease the set time of the adhesive bond. The waxes crystallize rapidly resulting in a step-change in viscosity during cooling and preventing movement of the bond. For good adhesion, the wax crystals are preferably as small as possible and preferably do not form a wax surface layer on the adhesive.
It is desirable to keep a continuous polymer phase in the bulk phase to enable elongation of the matrix under stress. If this is not done, the adhesive may break at very short extensions and not be able to absorb the energy during bond deformation. It is therefore desired to have an adhesive having small and/or homogeneously distributed domains as described herein.
Good surface phase structures or domains are also desirable. Many adhesives have good adhesion to certain substrates but not to others. For example, some adhesives do not exhibit high all-around performance on polar surfaces, such as polyethylene terephthalate (PET), acrylic varnishes or low energy surfaces such as polyethylene or fluorinated substrates. The adhesive industry recognizes this problem but has yet to achieve a solution. Background references include U.S. Pat. Nos. 4,554,304, 4,719,260, WO 03/025036, WO 03/025037, WO 03/025038, WO 03/025084, JP 52 090535 A, and EP 0 803 559 A.
Thus, a need exists for optimum adhesives that achieve good adhesive performance regardless of the substrate. There also exists a need for a method to determine which adhesive compositions might be candidates for excellent performance on multiple substrates.
While not wishing to be bound by any theory, Applicants believe that adhesive performance, including the debonding process, is dependent on the phase structures (also referred to herein as domains) in the bulk of the adhesive and on the surface of the adhesive. When the pull-off force domains or stiffness domains are below a certain area and/or have a good homogeneous domain distribution, the overall adhesive performance is believed to be better.